In the field of projection displays, a high intensity lamp is required which emits highly luminous fluxes from small areas. A widely used example is a xenon arc lamp which may emit 2000 lumens from a few square millimeters with an efficiency of 10 lumens/watt. The xenon arc lamp suffers from lack of high efficiency, a requirement for a high current, low voltage power supply which is expensive, and a lamp life which may not extend beyond 1000 hours.
Alternatives to the xenon arc lamp are conventional tungsten or gas discharge lamps. These lamps suffer similar problems in achieving high brightness (and efficiency) along with high output, yet are relatively small in dimension and provide close to point source light.
Van Tol et al in "A High Luminance High-Resolution Cathode-Ray Tube for Special Purposes" appearing in the IEEE Transactions on Electron Devices, Vol. ED-30, No. 3, March 1983 at pages 193-197 describes a light source consisting of a cathodoluminescent screen consisting of rare-earth doped yttrium-aluminum garnet (YAG) epitaxially grown on commercially available YAG substrates. Van Tol et al report that the arrangement provides relatively high efficiency, good brightness, and does not require a high current supply.
On the other hand, the epitaxial nature of the layer has a profound influence on the optical characteristics; in particular light trapping in the epitaxial layer severely reduces the useful emission. For example, the authors report that any light emitted at angles with the normal to the screen surface that are larger than the critical value will not even leave the screen until the light has travelled sideways to the edges of th screen, and in that event even if the light does leave the screen it may not leave it so as to be usefully directed.
It is thus one object of the invention to provide a high intensity point-like light source having particular utility for projection displays, which does not require high current supply, exhibits good brightness and efficiency, and is not limited by the light trapping effects reported by van Tol et al.
All embodiments of the invention described hereinafter use a self-supporting garnet crystal, preferably yttrium-aluminum garnet which is cerium doped, or activated, as the active light source. Nevertheless, it will be apparent that doping other than cerium could also be used. Various embodiments of the invention provide for the crystal in the form of a sphere, a rod, or a prismatic shape of rectangular or circular cross-section. Advantageously, the YAG crystal is in intimate contact with a heat sink. For certain embodiments of the invention, light trapping is advantageously employed by providing a metallic reflecting coating over a majorirty of the exposed surface of the crystal so that light is preferentially emitted through a selected area or region.
The intense luminescence of the YAG crystal can be achieved by electron excitation (as is the case in all the embodiments specifically described) although photo excitation is also contemplated.
The use of a self-supporting crystal eliminates the epitaxial growth complexity of the prior art and provides the lamp designer with additional freedom to select the geometry or the light emitter to achieve a desired effect. Light trapping in the crystal is used to advantage by selectively coating the crystal to provide for a preferential region (as well a direction) of light emission.